The invention relates to a process for producing pressurized nitrogen by low-temperature fractionation of air in a rectification system which has a pressure column and a low-pressure column, in the process, feed air being passed into the pressure column, an oxygen-containing liquid fraction being taken off from the pressure column and fed into the low-pressure column, gaseous nitrogen from the low-pressure column being at least partially condensed in a top condenser by indirect heat exchange with an evaporating liquid and nitrogen from the low-pressure column being produced as gaseous pressurized nitrogen product at a pressure which is higher than the operating pressure of the low-pressure column.
A process of this type is disclosed by DE 3528374 A1. Here, nitrogen produced in particular at the top of the low-pressure column is removed as pressurized product. In addition, the nitrogen is taken off in the gaseous state from the low-pressure column, heated in the main heat exchanger against feed air and then compressed from about low-pressure column pressure to the product pressure.
The object underlying the invention is to produce nitrogen at high pressure with relatively little expenditure.
This object is achieved by means of the fact that at least a part of the liquid nitrogen produced in the indirect heat exchange in the top condenser or liquid nitrogen withdrawn from the low-pressure column is brought in the liquid state to a pressure which exceeds the pressure of the low-pressure column, is evaporated in a product evaporator by indirect heat exchange with a heat-transfer medium and is produced as pressurized nitrogen product. The product evaporator can be disposed within one of the columns or outside the columns.
The pressure increase in the nitrogen product from the low-pressure column is therefore at least partially carried out in the liquid state. The pressure increase in the liquid can be carried out by any known measure, for example by means of a pump, utilization of a hydrostatic potential and/or pressurizing evaporation in a tank. It implies a lower expenditure on apparatus than a gas compressor. Indirect heat exchange is additionally required in which the low-pressure column nitrogen pressurized in the liquid state is evaporated. Nevertheless, this gives overall a particularly economically favourable process.
In comparison with take-off of the pressurized nitrogen product directly from the pressure column, the process according to the invention additionally has the advantage of higher product purity. In particular, in the low-pressure column, a concentration of more volatile components such as helium, neon and/or hydrogen can be achieved which is decreased in comparison with the top product of the pressure column. Preferably, in the invention all of the nitrogen product of the low-pressure column is taken off in the liquid state from the low-pressure column or from its top condenser.
The operating pressures of the double column in the process according to the invention can be for example 6 to 20, preferably 7 to 16, bar in the pressure column and, for example 3 to 8, preferably 3 to 6, bar in the low-pressure column. The top condenser of the low-pressure column is operated, for example, with a liquid from the low-pressure column, such as, for instance, the low-pressure column bottom-phase liquid, as refrigerant. Reflux for the pressure column is usually produced by a condenser/evaporator, via which the top of the pressure column and the bottom of the low-pressure column are in heat-exchanging connection.
There are two preferred possibilities for the choice of the heat-transfer medium for evaporating the low-pressure column nitrogen pressurized in the liquid state.
Firstly, a gas from the pressure column, preferably a nitrogen-containing fraction from an upper or central region of the pressure column, can be used as heat-transfer medium. This can be the top fraction of the pressure column or a gas which is withdrawn at an intermediate point of the pressure column. This intermediate point is situated below the pressure column top by a number of theoretical plates which is up to 5/6, preferably 1/3 to 5/16, of the total number of theoretical plates within the pressure column. The condensate produced in the indirect heat exchange in the product evaporator is recycled at least in part, preferably completely, back to the pressure column and there used as reflux.
Alternatively, or additionally, a gas from the low-pressure column is used as heat-transfer medium for evaporating the low-pressure column nitrogen pressurized in the liquid state, preferably an oxygen-containing fraction from a lower or central region of the low-pressure column. This can be the bottom-phase fraction of the low-pressure column or a gas which originates from an intermediate point of the low-pressure column. This intermediate point is situated above the low-pressure column bottom by a number of theoretical plates which is up to 5/6, preferably 1/3 to 5/6, of the total number of theoretical plates within the low-pressure column. The condensate produced in the indirect heat exchange in the product evaporator is recycled at least in part, preferably completely, back to the low-pressure column.
In addition, it is expedient if the liquid nitrogen only evaporates in part in the indirect heat exchange in the product evaporator and the portion of the nitrogen which remains liquid is returned to the low-pressure column. The product evaporator in this case is preferably operated as a falling-film evaporator. This type of evaporation makes a particularly low temperature difference possible and thus a correspondingly high evaporation pressure which, even when pure nitrogen from the top of the pressure column is used as heat-transfer medium, is only slightly (about 0.3 to 0.8 bar) below the pressure column pressure. The circulation pump used is the pump present in any case for pressure boosting; the low-pressure column serves as flash gas separator when the portion which remains liquid is recycled.
To produce refrigeration it is conventional to subject a process fraction to work-producing expansion. In the context of the invention it is advantageous if the energy produced in the work-producing expansion is used for further compression of the pressurized nitrogen product downstream of the product evaporator. The pressurized nitrogen product from the low-pressure column can thus be brought to pressure column pressure with low expenditure and mixed with nitrogen product withdrawn directly from the pressure column. The mixture can be used as product or compressed to a still higher pressure. The process fraction to be subjected to work-producing expansion can be a partial stream of the feed air, evaporated refrigerant from the top condenser of the low-pressure column or a gas from the lower region of the low-pressure column.
Usually, the bottom-phase liquid of the low-pressure column is used as refrigerant to condense the gaseous nitrogen from the low-pressure column in the top condenser of the low-pressure column. However, if in the context of the process according to the invention, in addition to the pressurized nitrogen, relatively pure or pure oxygen (purity greater than 40 mol %, in particular greater than 80 mol % or greater than 90 mol %, preferably between 99.5 and 99.999 mol %) is to be produced, it is particularly expedient if a liquid fraction whose oxygen content is between that of the oxygen-containing liquid fraction from the pressure column and that of the bottom-phase liquid of the low-pressure column, is used to condense the gaseous nitrogen from the low-pressure column in the top condenser. This can be the oxygen-containing liquid fraction from the pressure column itself or a liquid produced after its expansion to about low-pressure column pressure, or else a liquid fraction which is taken off from the low-pressure column above the bottom, but below the feed of the oxygen-containing liquid fraction. In this manner, a pure oxygen product can be taken off in the liquid and/or gaseous state from the lower region of the low-pressure column, more precisely at the superatmospheric pressure of the low-pressure column. The refrigerant for the top condenser of the low-pressure column nonetheless has a higher nitrogen content than the oxygen product and thus a relatively low evaporation temperature.